Electronic memory may be volatile memory or non-volatile memory. Non-volatile memory is able to store data when power is removed, whereas volatile memory is not. Resistive random access memory (RRAM) is one promising candidate for next generation non-volatile memory technology due to its simple structure and its compatibility with complementary metal-oxide-semiconductor (CMOS) logic fabrication processes. The RRAM, however, still suffers from narrow switching window and leakage issues.